H2—O2 fuel cells separate the hydrogen (H2) fuel and an oxidant, typically oxygen from air, with an electrolyte. Within the fuel cell, the hydrogen gas separates into electrons and hydrogen ions (protons) at the anode. The hydrogen ions pass through the electrolyte to the cathode with the electrons traveling through a power circuit (e.g., to a motor) and returning to the cathode, where they combine with the hydrogen ions and oxygen to form water. The reaction rates at the anode and cathode are generally enhanced by a catalyst.
There are several broad types of fuel cells, each incorporating a different electrolyte system, and each having advantages that may make them particularly suited to given commercial applications. One type is the proton exchange membrane (PEM) fuel cell, which employs a thin polymer membrane that is permeable to protons but not electrons. PEM fuel cells, in particular, are well suited for use in vehicles, because they can provide high power and weigh less than other fuel cell systems.
For many applications, it is desirable to use a readily available hydrocarbon fuel, such as methane (natural gas), methanol, gasoline, or diesel fuel, as the source of the hydrogen that will be feed into the fuel cell. Such fuels are relatively easy to store, and there is an existing commercial infrastructure for their supply. Due in part to the established production, storage and distribution infrastructure, liquid fuels such as gasoline are particularly suited for vehicular applications. However, hydrocarbon fuels must be dissociated to release hydrogen gas for use in the fuel cell. Power plant fuel processors for providing hydrogen contain one or more reactors or “reformers” wherein the fuel reacts with steam, and sometimes air, to yield reaction products comprising primarily hydrogen and carbon dioxide.
The use of hydrocarbon reformate fuel cell systems in cars and other vehicles presents special concerns. In addition to the desirability of using readily available liquid fuels, discussed above, the reformer and fuel cell systems must be relatively light in weight, and must be able to operate efficiently under a wide range of ambient conditions (e.g., under a range of temperatures and humidity conditions). They should also exhibit good cold-start performance to produce power quickly, and respond quickly to varying system demands to provide the necessary power quickly. Thus, it is desirable to minimize the need for external heating of the reactants being fed into the reformer. It is also desirable to minimize the amount of liquid water that must be supplied to or handled within the system, to reduce or avoid the need to replenish system water and to reduce the complications associated with operations at temperatures below 0° C. (32° F.).
Typically, there are several components in the reformate fuel cell system that require water, particularly including the reformer (e.g., a steam reformer or autothermal reformer) that requires steam as a reactant and some carbon monoxide clean-up reactors (e.g., a water gas shift or WGS reactor), as well as the fuel cell that requires humidification of the MEA in order to function properly. A common approach to enhancing water balance in fuel cell systems incorporates a series of condensing heat exchangers at various points in the system. For example, a condensing heat exchanger may be positioned downstream of the reformer to cool the reformate to a temperature at or below its dew point and thereby condense a portion of the water vapor. The condensate water is then separated from the gaseous reformate and stored in a reservoir until it is returned to the reformer where it is heated to create steam. Heat exchangers have also been used to cool the cathode exhaust stream and condense water vapor that can then be used to humidify the MEA.
The use of multiple heat exchangers increases the complexity of the resulting reformer system. For example, the water recovery efficiency of heat exchangers is reduced as the ambient temperature increases. Similarly, large radiators may be required to dissipate the heat of condensation. Moreover, the liquid condensate produced by the heat exchangers must be vaporized before being fed back into the reformer or fuel cell, thereby creating an additional energy load and decreasing the overall efficiency of the system.
Various methods for addressing the water balance within fuel cell systems have been described in the art. See, for example, German Patent Disclosure 42 01632, Strasser, published Jul. 29, 1993; U.S. Pat. No. 6,007,931, Fuller et al., issued Dec. 28, 1999; and U.S. Pat. No. 6,013,385, DuBose, issued Jan. 11, 2000. However, water management systems among those known in the art do not adequately address these needs, due to problems such as their inability to maintain true water balance over a wide range of operating conditions, mechanical complexity, reliability concerns, and increased system energy requirements.